Background of Section I
The patterning process in semiconductor manufacturing involves several steps, including mask manufacturing, optical exposure, resist processing, and reactive ion etching. Models can be used to describe, for example, lens aberrations present in the optical exposure step. These models are built by measuring various characteristics of a pattern exposed across a wafer. In practice, a pattern is exposed on the wafer, the wafer is stepped, and the pattern is exposed onto a different part of the wafer, such that the exposure fields are non-overlapping. This process is repeated several times across the wafer. Various characteristics of the exposed patterns are then measured to generate data needed to build the model. One source of error in building a model in this way is that a given characteristic can have different measured values depending on the characteristic's location on the wafer.
Several approaches have been described to reduce the effect of this across-wafer variation. U.S. Pat. No. 6,646,729 to Van der Laan et al. describes a methodology for aberration measurements in which across-wafer variation is circumvented by using a technique referred to as “micro-stepping” or “die-in-die” exposures. In that technique, the same part of the wafer is used so that all exposures are subject to substantially identical errors. Similarly, U.S. Pat. No. 6,130,747 to Nomura et al. describes a micro-stepping methodology in which the exposures needed for an assessment of astigmatism aberration is performed by placing exposures of similar features in close proximity through multiple exposures that are placed in slightly different locations. See also U.S. Pat. No. 6,091,486 to Kirk.
Background of Section II
This invention relates to a method of characterizing lithography projection equipment used in manufacturing of integrated circuits.
Referring to FIG. 1, a conventional optical lithographic scanner system, for image-wise exposure of a coating of resist on a semiconductor wafer, includes an extended light source for emitting a beam of actinic radiation. The illuminator optics project the light from the extended source onto an exposure mask (also called a reticle) that defines a pattern of features that are to be transferred from the mask to the resist coating. Apertures inserted at the extended light source define the angular light distribution of the illuminator. The light passes through the mask, which is shown as having a single opaque element in a transparent field. The projection optics create an image of the mask pattern on the resist coating. Upon exposure to radiation chemical reactions are initiated within the resist that ultimately change the resist solubility in aqueous solutions. In order to complete the reactions initiated during exposure elevated temperatures are required and therefore the wafer is placed on a bakeplate for a certain amount of time. This process step is commonly referred to as post exposure bake. The solubility determines the speed with which resist is removed if covered by an aqueous solution (typically a solution of TMAH in water). The process of removing resist in the areas that have been exposed to light is referred to as resist development. High solubility corresponds to a fast development rate (i.e. removal rate). The solubility change depends in a highly nonlinear, approximately step-like fashion on the radiation dose. There are two categories of dose dependencies. For example there are resists which have very low to negligible development rates at low doses and their development rate increases with increasing dose. Such resists are referred to as positive resists. The inverse relationship, high development rate at low doses and small development rate at large doses, is found in negative resists. Resist thickness is a variable that can easily be measured with existing equipment in the semiconductor manufacturing line. Therefore a common way of representing the dependency of the development process on exposure dose in graphic form is to plot the remaining resist thickness as a function of exposure time given a fixed initial resist thickness and time of development. Even more easily detectable than the resist thickness is the lowest dose at which the resist is completely removed from the wafer. The corresponding dose is referred to as ‘dose to clear’ (D0). The value of D0 is not a fixed property of the resist, rather it depends on a variety of process parameters, some of which have already been mentioned (resist thickness, develop time). Other parameters are for example the temperature of the post exposure bake, time delay between exposure and bake as well as developer concentration and temperature. These parameter may vary across the wafer and thus lead to across wafer variations of the dose to clear D0. A common cause of such variations is nonuniform temperature distributions across the post exposure bake plate.
After the development process, a pattern of resist features that corresponds to the pattern of opaque features of the exposure mask (in the case of a positive resist) remain on the wafer. This resist pattern may then be transferred into the underlying substrate through various etch processes as areas covered by resist are not subject to the etch erosion.
FIG. 1 provides a schematic overview of an imaging system. Light from an extended light source is projected onto the reticle with the illuminator optics. The pattern created by those areas which block light and those which allow light to pass through the projection optics is imaged onto the resist pattern covering the wafer. The best image characterized by the highest fidelity of the imaged pattern to the original mask pattern exists only for a specific distance between projection optics and wafer and to a lesser degree for a small range around this distance. For other distances other characteristics of the imaging system may determine the pattern formed on the wafer and the pattern may not at all be reminiscent of the pattern on the mask.
The transfer of the complete mask image onto the wafer may be accomplished via one single exposure, in which case the full image area of the mask is uniformly illuminated at once. In this case the dose delivered to the wafer is controlled by opening a shutter for a certain amount of time. Alternatively, only a slit-shaped region of the mask is exposed at a time projecting an equivalent image on the wafer. Image transfer of the full mask image is accomplished through simultaneous, highly coordinated movement of both reticle and wafer stage, a process referred to as scanning. The dose delivered in this case is largely controlled via the speed of this scanning movement. The maximum area of the wafer that can be exposed in a single exposure is referred to herein as the exposure field. The exposure field corresponds to the area of the wafer that would be exposed in the event that the mask were completely clear, and its size and shape are therefore governed by the exposure tool. Generally, the exposure field is rectangular and the exposure tool is used to expose multiple exposure fields that are adjacent each other on a rectangular grid. Thus, to expose an entire wafer, in both imaging approaches once a first exposure field has been exposed the stage is stepped to a new position followed by the next field exposure. In normal operation each field on the wafer is only exposed once, however there are several applications where it is advantageous to expose a field more than once, in most cases with different masks. These techniques are called double exposure techniques. The exposure tool is not limited to exposing exposure fields that are in abutting relationship. For example, the stepping distance along the X or Y axis may exceed the size of the rectangular exposure field, in which case there will be guard strips between adjacent exposure fields, or the stepping distance along either or both axes may be less that the size of the rectangular exposure field, in which case the exposure fields would overlap.
The area of the wafer that is exposed by a bright feature of an exposure mask is referred herein as the image field. A mask may have several discrete bright features, in which case there are, correspondingly, several image fields on the wafer.
For modern lithography systems, controlling the characteristics of the illumination system has become an important parameter in the lithographic process setup. Modern illuminators not only achieve uniform illumination intensity across the mask, but they also provide control over the angular light distribution that impinges on each point of the reticle. Examples of such distributions are shown in FIG. 2, which depicts examples of conventional homogeneous and annular illumination. This figure represents common illumination patterns employed in lithographic systems. For example in conventional homogeneous illumination each point on the reticle field is illuminated with light up to a certain angle of incidence. Therefore if each ray of light passing through the reticle at a given point is drawn, the resulting shape is that of a cone as shown in FIG. 2. In the case of annular illumination, only light within a certain range of angles strikes the reticle. The corresponding representation therefore appears as a double cone. The limiting angles shown in these images are given as ratios referred to as the sigma value. Sigma is defined as the ratio between the sine of the angle on the illuminator and the numerical aperture NA (which in itself is the sine of an angle). The most common way of providing a graphical representation of the actual performance of an illuminator is to use the sine of the angle of incidence and the rotational angle in the plane, to identify a certain direction and plot the intensity delivered by the illuminator in this direction. The resulting graphs are 3D representations of the illuminator performance sometimes also referred to as ‘pupilograms’ or ‘pupil illumination’ as it describes the light distribution at the entrance pupil of the projection system in the case that there is no reticle present. This distribution will be referred to as intensity distribution of the illuminator, or illumination distribution for short. Knowledge of this distribution is of great significance as it plays a key role in determining the imaging performance of the projection system.
Given these distributions it is now apparent that a variety of other non-uniformities and non ideal illumination patterns are possible. For example, each one of the directions indicated in FIG. 2 may have different intensities associated with it resulting for example in non-radially symmetric distributions. As another example, the transition at the limiting angle may not be as sharp as implied in the figure and there may be a more gradual transition from zero intensity at some angle larger than the cutoff angle indicated in FIG. 2 to the desired intensity value at some angle smaller than the cutoff angle. Such imperfections lead to variations and non ideal behavior in the imaging performance of an exposure system. For example, asymmetric illumination patterns result in pattern asymmetries as a function of focus. The exact shape of the illuminator distribution also controls other important imaging characteristics such as the difference in resist linewidth between isolated and nested features and line end foreshortening.
Finally all these characteristics may vary from one point on the reticle, or exposure, field to another.
As a result, several techniques have been developed to characterize the illumination system. J. P. Kirk et al. “Pupil Illumination; in situ measurement of partial coherence”, Proc. SPIE Vol. 3334, 1998, p. 281-288 describes a technique for recording the illumination distribution. In this technique an obscuration (negative pinhole) is placed on the backside of a reticle. The image of such a feature is formed at a distance far away from the wafer plane. As a result the pattern formed on the wafer is representative of the illumination distribution rather than the shape of the obscuration. Several resist images at a series of exposure doses are recorded in resist and allow a reconstruction of the illumination distribution. Brunner et al., U.S. Pat. No. 6,048,651 discloses a modification of the general methodology using a fresnel zone target as the obscuration.
B. B. McArthur et al., U.S. Pat. No. 6,356,345 “In Situ Source Metrology Instrument and Methodology of Use” discloses a methodology for determining pupil illumination by providing a set of field points in the object plane together with an array of aperture planes at a sufficient distance from the reticle such as to provide an image of the illumination. In one of the best mode implementations the images are also recorded in resist.
Double exposure techniques have been employed in lithography and to some extent in scanner characterization. In particular we refer to the co-pending patent application Ser. No. 10/933,090 filed Sep. 1, 2004. In addition F. Zach et al. “Aberration Analysis using Reconstructed Aerial images of Isolated Contacts on Attenuated Phase shift masks” describe a double exposure method for determining aberrations of an exposure tool. In this publication images of a contact hole are superimposed onto a first, uniform exposure with a dose of less than dose to clear. Based on an analysis of the image intensity in the sidelobe of the main contact image, aberrations can be extracted.